[LSR] Don't consider users of constant outside loop

[nikic]

Summary

In CollectLoopInvariantFixupsAndFormulae(), LSR looks at users outside the loop. E.g. if we have an addrec based on %base, and %base is also used outside the loop, then we have to keep it in a register anyway, which may make it more profitable to use %base + %idx style addressing.

This reasoning doesn't hold up when the base is a constant, because the constant can be rematerialized. The lsr-memcpy.ll regressed when enabling opaque pointer, because inttoptr (i64 6442450944 to ptr) now also has a use outside the loop (previously it didn't due to a pointer type difference), and that extra "use" results in worse use of addressing modes in the loop. However, the use outside the loop actually gets rematerialized, so the alleged register saving does not occur.

This patch opts to skip users of constants entirely for this heuristic. I considered a narrower fix for only inttoptr expressions first, but as this doesn't impact any other tests, it seems better to just handle all constants.

[dmgreen]

Sounds OK to me. If you are changing all Constant's it might be worth adding a test for globals too, they are likely to be more common. I believe the same logic should apply to them too.

[nikic]

Add test variant with globals.

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

while (!Worklist.empty()) {
     else if (const SCEVUDivExpr *D = dyn_cast<SCEVUDivExpr>(S)) {
       Worklist.push_back(D->getLHS());
       Worklist.push_back(D->getRHS());
     } else if (const SCEVUnknown *US = dyn_cast<SCEVUnknown>(S)) {
       const Value *V = US->getValue();
       if (const Instruction *Inst = dyn_cast<Instruction>(V)) {
         // Look for instructions defined outside the loop.
         if (L->contains(Inst)) continue;
-      } else if (isa<UndefValue>(V))
-        // Undef doesn't have a live range, so it doesn't matter.
+      } else if (isa<Constant>(V))
+        // Constants can be re-materialized.
         continue;
       for (const Use &U : V->uses()) {
         const Instruction *UserInst = dyn_cast<Instruction>(U.getUser());
         // Ignore non-instructions.
         if (!UserInst)
           continue;
         // Don't bother if the instruction is an EHPad.
         if (UserInst->isEHPad())

llvm/test/Transforms/LoopStrengthReduce/AArch64/lsr-memcpy.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 2
 ; RUN: llc -mtriple=arm64-unknown-unknown -mcpu=cyclone -pre-RA-sched=list-hybrid < %s | FileCheck %s
 ; rdar://10232252
 ; Prevent LSR of doing poor choice that cannot be folded in addressing mode
 
 ; Remove the -pre-RA-sched=list-hybrid option after fixing:
 ; <rdar://problem/12702735> [ARM64][coalescer] need better register
 ; coalescing for simple unit tests.
 
-; FIXME: This regressed after enabling opaque pointers.
 define i32 @test_inttoptr() nounwind {
 ; CHECK-LABEL: test_inttoptr:
 ; CHECK:       // %bb.0: // %entry
 ; CHECK-NEXT:    str x30, [sp, #-16]! // 8-byte Folded Spill
-; CHECK-NEXT:    mov x8, #0 // =0x0
-; CHECK-NEXT:    mov w9, #1288 // =0x508
-; CHECK-NEXT:    mov x10, #4294967296 // =0x100000000
-; CHECK-NEXT:    mov x11, #6442450944 // =0x180000000
+; CHECK-NEXT:    mov w8, #1288 // =0x508
+; CHECK-NEXT:    mov x9, #4294967296 // =0x100000000
+; CHECK-NEXT:    mov x10, #6442450944 // =0x180000000
 ; CHECK-NEXT:  .LBB0_1: // %while.body
 ; CHECK-NEXT:    // =>This Inner Loop Header: Depth=1
-; CHECK-NEXT:    ldr x12, [x8, x10]
-; CHECK-NEXT:    str x12, [x8, x11]
-; CHECK-NEXT:    add x8, x8, #8
-; CHECK-NEXT:    subs x9, x9, #8
+; CHECK-NEXT:    ldr x11, [x9], #8
+; CHECK-NEXT:    str x11, [x10], #8
+; CHECK-NEXT:    subs x8, x8, #8
 ; CHECK-NEXT:    b.pl .LBB0_1
 ; CHECK-NEXT:  // %bb.2: // %while.end
 ; CHECK-NEXT:    mov x8, #6442450944 // =0x180000000
 ; CHECK-NEXT:    blr x8
 ; CHECK-NEXT:    mov w0, #0 // =0x0
 ; CHECK-NEXT:    ldr x30, [sp], #16 // 8-byte Folded Reload
 ; CHECK-NEXT:    ret
 entry:

 }
 
 @g1 = external dso_local global i8
 @g2 = external dso_local global i8
 
 define ptr @test_globals() nounwind {
 ; CHECK-LABEL: test_globals:
 ; CHECK:       // %bb.0: // %entry
-; CHECK-NEXT:    mov x8, #0 // =0x0
-; CHECK-NEXT:    mov w9, #1288 // =0x508
-; CHECK-NEXT:    adrp x10, g2
-; CHECK-NEXT:    add x10, x10, :lo12:g2
-; CHECK-NEXT:    adrp x11, g1
-; CHECK-NEXT:    add x11, x11, :lo12:g1
+; CHECK-NEXT:    mov w8, #1288 // =0x508
+; CHECK-NEXT:    adrp x9, g2
+; CHECK-NEXT:    add x9, x9, :lo12:g2
+; CHECK-NEXT:    adrp x10, g1
+; CHECK-NEXT:    add x10, x10, :lo12:g1
 ; CHECK-NEXT:  .LBB1_1: // %while.body
 ; CHECK-NEXT:    // =>This Inner Loop Header: Depth=1
-; CHECK-NEXT:    ldr x12, [x10, x8]
-; CHECK-NEXT:    str x12, [x11, x8]
-; CHECK-NEXT:    add x8, x8, #8
-; CHECK-NEXT:    subs x9, x9, #8
+; CHECK-NEXT:    ldr x11, [x9], #8
+; CHECK-NEXT:    str x11, [x10], #8
+; CHECK-NEXT:    subs x8, x8, #8
 ; CHECK-NEXT:    b.pl .LBB1_1
 ; CHECK-NEXT:  // %bb.2: // %while.end
 ; CHECK-NEXT:    adrp x0, g1
 ; CHECK-NEXT:    add x0, x0, :lo12:g1
 ; CHECK-NEXT:    ret
 entry:
   br label %while.body